Hein has 4,000,000 HP, which is almost 10 times what he has on the normal event. As typical of Hein, he will begin the battle using his signature Barrier Shift to modify his weakness, and rotate through them for the rest of the battle. During his first turn he will always set it to be weak to Lightning, but will later rotate between Fire and Ice weaknesses as well. He can also use another version of Barrier Shift that affects the party's parameters, by inflicting ATK/DEF or MAG/SPR debuffs or modifying their weaknesses.
Hein can attack up to 7 times per turn, usually using Magic attacks with normal attacks. He can use Firaga, Thundaga, Blizzaga, to damage all the party. He also uses Confuse and Sleep on single targets but with 100% success of inflicting either ailment without protection. He can also use Bioga for Dark Damage and guaranteed Poison, but does so every 2 turns.
Upon reaching 50% threshold he uses Thundaga and will become more aggressive on his use of Spells.
Hein resists all elements by 200% unless affected by Barrier Shift. He's resistant to Breaks except SPR and is immune to all status ailments except Disease.
The battle can be tricky and will need some careful preparation. His high 9,999 DEF value still sticks, so Physical attacks will be useless, unless they are the kind that uses Magical Damage. As the usual norm, the player will have to capitalize on Magic Damage to triumph.
The tricky part of this is being capable of dealing enough damage to defeat Hein within 15 turns while being capable to deal with his powerful attacks and status ailments. His physical attacks are also bound to be damaging to Mages who often have frail DEF, all the while trying to keep a party of 5 members at most.
Given all of this the player will have to split between attack and survival. Marie although a rare unit, is an excellent choice for this battle as her Love You All and Lovely Guard will protect against elemental damage and status ailments respectively. If enhanced they come with additional benefits such as a large Regen and defensive buffs. Additionally, Celes' Sealing Blade and the Manufacted Nethicite can absorb some of his Spells.
Ashe also works as her Heaven's Fury attack can provide heals and build chains, but will need a copy of herself to perfect chain. Being neutral damage it will circumvent Hein's defenses. More streamlined mages like Exdeath or Rem are also helpful as they can capitalize on Hein's weaknesses. Rem can double as a healer and her Dagger Boomerang can be used twice and build power for other moves.
Abusing Magic attacks will likely deprive the player of MP rather quick, so an MP Battery should be used. The recently enhanced Charlotte can be useful as she can protect against Magic damage, and restore MP. Ling can also be used as she can inflict debuffs, restore MP, and is herself immune to status ailments and can remove them if needed. If available Lunera is also fantastic, as her Elven Song provides stat buffs as well as great HP/MP Regen.
If Magic Damage strategy proves to be difficult, the player can rely on the usual Setzer strategy. Since Double Dice ignores Hein's DEF it will be able to deal solid damage and capitalize on Killer passives for further damage (elemental weapons should be avoided however). If using this strategy, the player can chain with physical attacks to increase the possible damage from Double Dice, since this skill will be the main damager it is irrelevant that other attacks do not.
Damage Reduction abilities like Defensive Barrier are helpful, as they can cover both physical and magic damage. Building HP through materia or equips is also a good choice. The one thing the player has to be wary is to make sure their units are immune to the status ailments as soon as possible as having to waste turns healing either will endanger the player's chances to beat Hein under 15 turns.
If doing all Missions in one go is too difficult, the player should try defeating Hein with a party of 5 and ignore the number of turns, and then go with a party of 6 and attempt to beat him under 15 turns, as these are likely to be the most difficult.